[llvm-dev] [cfe-dev] Should isnan be optimized out in fast-math mode?

Wed Sep 15 10:42:26 PDT 2021

Fundamentally, the question Serge asked has nothing to do with the concerns of “compiler people”, and everything to do with the user facing behavior of the compiler. Any talk of how the behavior should be implemented is (in my opinion) off topic until we settle the question of “should the compiler guarantee, as a special case, that isnan(x) will not be optimized out”. This is a Yes-or-No question, and the explanation for the answer needs to be able to be concisely described in the docs without the use of compiler jargon that Joe GameDev and Tom MLScientist probably don’t understand. I have stated my opinion, and am reluctant to wade into this argument again, but I think it’s important that we understand the issue at hand.

There are two productive outcomes of this question that I can see:

  1.  The answer is No. In this case, we just need a small doc fix
  2.  The answer is Yes. Only in this case do we actually need to modify the implementation

Let’s not put the cart before the horse. The “compiler people” don’t necessarily have to do anything.

Thanks,
   Chris Tetreault

From: cfe-dev <cfe-dev-bounces at lists.llvm.org> On Behalf Of Krzysztof Parzyszek via cfe-dev
Sent: Tuesday, September 14, 2021 3:28 PM
To: Arthur O'Dwyer <arthur.j.odwyer at gmail.com>; Serge Pavlov <sepavloff at gmail.com>
Cc: llvm-dev at lists.llvm.org; cfe-dev at lists.llvm.org
Subject: Re: [cfe-dev] [llvm-dev] Should isnan be optimized out in fast-math mode?

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Anyway, Richard's "quiet is signaling and signals are unspecified values" is really the only way out of the difficulty, as far as compiler people are concerned. You two (Serge and Krzysztof) can keep talking past each other at the application level, but the compiler people are going to have to do something in the code eventually, and that something is going to have to be expressed in terms similar to what Richard and I have been saying, because these are the terms that the compiler understands.

I don’t know why you’re saying “at the application level”.  My concerns are motivated by what the compiler is supposed to do.  I don’t think that the consequences of “arithmetic operations don’t produce NaNs” are fully understood, and are likely not completely intuitive either.  We may end up having discussions as to whether we should optimize x+0 to x or not, because “x+0” carries the information that it won’t result in a NaN, while “x” alone doesn’t.  This is one case that comes to mind and I’m concerned that there are many others that we aren’t aware of yet.

--
Krzysztof Parzyszek  kparzysz at quicinc.com<mailto:kparzysz at quicinc.com>   AI tools development

From: Arthur O'Dwyer <arthur.j.odwyer at gmail.com<mailto:arthur.j.odwyer at gmail.com>>
Sent: Tuesday, September 14, 2021 10:15 AM
To: Serge Pavlov <sepavloff at gmail.com<mailto:sepavloff at gmail.com>>
Cc: Krzysztof Parzyszek <kparzysz at quicinc.com<mailto:kparzysz at quicinc.com>>; llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>; cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] [llvm-dev] Should isnan be optimized out in fast-math mode?

WARNING: This email originated from outside of Qualcomm. Please be wary of any links or attachments, and do not enable macros.
On Tue, Sep 14, 2021 at 9:22 AM Serge Pavlov via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
On Tue, Sep 14, 2021 at 8:21 PM Krzysztof Parzyszek <kparzysz at quicinc.com<mailto:kparzysz at quicinc.com>> wrote:
If `has_nan` returns "true", it means that the explanation "there are no NaNs" does not work anymore and something more complex is needed to explain the effect of the option. In this case it is difficult to say that this approach is "intuitively clear".

If your program has “x = *p”, it means that at this point p is never a null pointer.  Does this imply that the type of p can no longer represent a null pointer?

Good example! If you use integer division `r = a / b`, you promise that `b` is not zero. It however does not mean  that preceding check `b == 0` may be optimized to `false`.

In C and C++, it actually does mean that, although of the compilers I just tested on Godbolt, only MSVC seems to take advantage of that permission.
https://godbolt.org/z/11ss5T7e8

The question of whether it is acceptable to treat as equivalent the statements "p is known to be dereferenced in all successors of B" and "p is known to be non-null in B," was discussed extensively about 20 years ago, and then again 12 years ago when it bit someone in the Linux kernel:
https://www.gnu.org/software/gcc/news/null.html
https://lwn.net/Articles/342330/
https://lwn.net/Articles/342420/
https://qinsb.blogspot.com/2018/03/ub-will-delete-your-null-checks.html

On Mon, Sep 13, 2021 at 10:28 PM Arthur O'Dwyer <arthur.j.odwyer at gmail.com<mailto:arthur.j.odwyer at gmail.com>> wrote:
Btw, I don't think this thread has paid enough attention to Richard Smith's suggestion:
I can only subscribe to James Y Knight's opinion. Indeed, it can be a good criterion of which operations should work in finite-math-only mode and which can not work. The only thing which I worry about is the possibility of checking the operation result for infinity (and nan for symmetry). But the suggested criterion is formulated in terms of arguments, not results, so it must allow such checks.

What is the opinion to which you subscribe?

Anyway, Richard's "quiet is signaling and signals are unspecified values" is really the only way out of the difficulty, as far as compiler people are concerned. You two (Serge and Krzysztof) can keep talking past each other at the application level, but the compiler people are going to have to do something in the code eventually, and that something is going to have to be expressed in terms similar to what Richard and I have been saying, because these are the terms that the compiler understands.

Thanks,
Arthur
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